This invention relates to a method for avoiding impingement between an acetabular cup and a femoral component; a femoral component and bone; or bone and bone.
Hip surgery requires the implantation of a femoral stem and an acetabular cup. The femoral stem has a spherical head that attaches to the neck of the stem and is free to articulate within a bearing insert that is fitted into the shell of an acetabular cup. Should the stem and cup not be positioned/aligned accurately, the neck of the stem may impinge on the lip of the insert resulting in a levering action that could allow the femoral head to cam out of the insert resulting in a permanent dislocation of the head. The impingement can also lead to excessive component wear and possibly failure.
In addition, a malpositioned cup can result in excessive liner wear even without impingement. A shell with a high abduction (inclination) angle can have joint forces concentrated near the cup liner rim, thereby increasing the wear rate due to concentrated forces.
A person's pelvis and two femurs form a “matched set” allowing the person to articulate one with the other under normal motions such as walking, squatting, etc., without dislocation. This matched set is different in every individual. One person may have a natural pelvic position such that it is flexed forward when standing. Another may have a pelvis that is flexed backward when standing. The amount of pelvic flexion is also referred to as pelvic tilt. Likewise, the amount of femoral neck version that matches with the pelvis will be different for each individual.
Stem/cup impingement can serve as a fulcrum that could lever the femoral head out of the cup, which is called dislocation, a serious clinical issue. Orthopaedic surgeons attempt to place an acetabular cup in the pelvis in an orientation that will hopefully not result in a stem/cup impingement. In placing the cup, it would help if the surgeons knew what the natural pelvic tilt and stem version were, therefore they could try to recreate it. However, this data, for the most part, is not currently available to the surgeon to take into consideration.
Reducing or eliminating the chance for neck/insert impingement and high inclination angles is critical to eliminating dislocation and wear, which can ultimately result in a revision surgery to correct compound alignment (abduction and anteversion).
The method of the present invention allows the surgeon to choose motions that matter (e.g. squatting for Asian population or gardeners). The surgeon can decide on implant types based on motions (e.g. a large femoral head vs. a smaller head, a femoral stem with different neck versions . . . ). The surgeon will know the boundaries of adjusting cup position, which is important to balance need to avoid iliopsoas tendon impingement with cup rim and high-wear inclination angles.
A study by H. Malchau et al., Clin. Orthop. Relat. Res. (2011) 469; 319-329 reviewed the implanted acetabular cup position post implantation in relation to the pelvic anatomy. The study of 1823 patients revealed that the cup position varied widely in reference to their described target zone.
One main reason for such variation is that the exact position of the patient's pelvis is not known in relation to the operating room (OR) table. Surgeons must rely on their experience to know how to position the cup, however the cup may not be implanted in the intended orientation. This is especially true with respect to less experienced surgeons.
Alignment of an acetabular cup can be achieved with an alignment guide that attaches to an insertion rod for facilitating the insertion of the acetabular cup into the acetabulum. The alignment guide preferably references the surgical table on which the patient rests. Conventionally, it is assumed that the patient's pelvis is parallel to the table, and that the surgical table is parallel to the floor. Based on such assumptions, the ordinary position (in most patients) for the acetabular cup is 45° of inclination (abduction) and 20° of anteversion. For a discussion of angles of anteversion and also inclination or abduction of the acetabular cup when installed in the acetabulum, see, for example, U.S. Pat. No. 6,395,005, the disclosure of which is incorporated by reference herein in its entirety.
It has been found based on post-operative x-rays, however, that despite the alignment guide being parallel to the floor during insertion, of the acetabular cup, the resultant inclination or anteversion of the acetabulum in relation to the alignment guide is often different than expected and, thus, the acetabular cup has been installed at a less than ideal position. The pelvic position changes in relation to the operating room table which is not recognized during the procedure for example.
Presently most orthopedic companies offer instrumentation to direct reaming for acetabular cups and cup impaction which is an antenna-like device that attaches to either the reamer shaft or the cup impaction tool. Such cup alignment instruments are shown in, for example, U.S. Pat. Nos. 5,037,424, 5,571,111 and 6,395,005. When an x-shaped “antenna” is used a cup impactor that is oriented 45 degrees to the floor and 20 degrees to the long axis of the patient. The ‘X’ shape on the antenna is set parallel to the floor, and one leg of the ‘X’ set in line with the long axis of the body. One leg is for a left leg operation, and the other for a right leg operation. Manual hip acetabular cup placement instruments use the operating table or floor as a reference with the assumption that pelvic tilt does not matter. Almost all surgeons place the acetabular cup in first and the stem second, therefore not taking into account the stem version which cannot be adjusted to the cup position.
Some of the drawbacks of this type of instrument are that the pelvis usually shifts when the patient is laid on the operating room table. If the pelvis does not shift, and the surgeon wants a 45/20 cup position, then the surgeon could use the instrument as is and get the perfect 45/20 alignment within the bone. However, most times the pelvis does shift in three possible planes: tilt, obliquity, and rotation. The surgeon does not know in which direction or by how much and therefore must use his experience or intuition to apply a correction factor to the direction of cup impaction. The actual cup orientation after impaction is not usually known until after the operation is complete and a post-operative x-ray is taken, and the patient is in recovery, and therefore at a time when changes to cup orientation are not possible without reoperation.
Another drawback is that the current antenna/impactor combination is set at set angles. For example a 45/20 degree abduction/anteversion orientation. If the surgeon determines that orientation of 40° to the floor and 15° to the long axis of the patient's femur is best for the patient, the set angles are of little use, or again the surgeon has to estimate a correct alignment. The orientation of the antenna/impactor combination in practice is set visually. The antenna shaft is set vertically, with the antenna ‘X’ cross bars parallel to the floor. The 20 degree orientation to the patient long axis is visual as well. Many surgeons do not use the antenna at all.
Some major prosthetic hip joint companies offer a navigation option to surgeons. This system uses cameras in the operating room and optical trackers on instrumentation. From a clinical perspective, the major drawbacks for navigation are that the technique involves placing invasive pins having the tracker thereon in the patient pelvis and femur. The pins are placed in the pelvis and the femur, through the skin and screwed into the bone. Using pins results in multiple separate wounds and increases the possibility of infection. This technique is also time intensive. Pins must be placed and pointers with trackers on them are used multiple times to register anatomy. This technique has a learning curve. The software and technique require extensive training and practical experience. Some systems require a pre-op CT scan which is costly. Navigation systems assume that the pelvis is in the same orientation for all cases.
A more recent development is digital imaging which produces an x-ray like image on a digital receiver. Once digitized, the digital image can be used to identify points in order for the system to calculate lengths and angles which could be used by the surgeon to help to identify how the pelvis is oriented to the operating room table. The surgeon can take pre-operative and intra-operative x-rays and pick points on the screen to calculate lengths and angles. This is a relatively new technology with a relatively small amount of users.
The digital x-rays can be visually observed for comparison. The system can aid the user by allowing the user to plan by designating the desired cup inclination and version angles pre-operatively as well as taking dimensions that will help to designate leg length and femoral stem offset corrections. Taking dimensions that will measure the cup inclination and version angles of the actual implanted cup intra-operatively as well as taking dimensions that show that actual leg length and offset of the trials or implants.
These current digital imaging systems do not have an algorithm that tries to compare pre-operative and intra-operative images to calculate how the intra-operative pelvic position changes in orientation to a pre-operative image. Furthermore, the current digital systems don't calculate the cup impaction angles that would account for these changes. Instead the cup needs to be first impacted into the bone prior to taking the image, and reoriented if not in the desired position. Reorienting the shell could compromise the fit and security of the cup to the acetabular bone cavity. Multiple reorientations could possibly compromise the fit to the point that a secure fit is no longer achieved. In this situation, the surgeon may have to remove the shell, ream up to the next size shell, and start over. The removal of further acetabular bone is not ideal as this could compromise the overall strength of the remaining bone, and reduce the amount of bone for any future revisions. Current digital imaging techniques require successive intra-operative images, exposing the patient and the surgical team to higher levels of radiation than with a single image.
U.S. Publication No. 2015/0088145, the disclosure of which is incorporated herein in its entirety, uses a combination of digital imaging and orientation technology to improve upon the limitations described above, along with an algorithm that determines the amount of pelvic movement in the three planes (obliquity, tilt, rotation) that is used as input for the orientation technology. U.S. Publication 2015/0088145 calculates pelvic tilt obliquity and rotation intra-operatively which no other system does. Most pre-operative x-ray images are taken lying down, and hence placing the pelvis in an unnatural position. A standing x-ray is the gold standard as the amount of pelvic tilt when standing is what is right for that individual. This invention serves to recreate the natural standing x-ray tilt and, if desired, obliquity and rotation amount intra-operatively by adjusting the orientation of reaming and cup impaction, and performing these functions at the pre-op plan angles determined by the user (e.g. at 40° inclination and 15° anteversion).
U.S. Publication No. 2015/088145 uses a method for aligning an acetabular cup including: taking a pre-operative preferably standing anterior/posterior view digital x-ray image and a lateral view standing digital x-ray image of the pelvis of a patient. A desired cup abduction and anteversion angle is determined based on two standing x-ray images. At least three points on the digital anterior/posterior digital x-ray image and at least two points on the lateral digital x-ray image are identified. The lengths and angles between each of the points on both the anterior/posterior digital image and the lateral digital image are calculated. A patient is positioned on an operating table in an operating room. The preferred operating table has a reference element thereon. In the preferred embodiment, the operating room has a navigation system therein, the preferred operating table has a navigation tracker mounted in a known position with respect to the operating table. Alternately, the reference system could reference the floor or other fixed point including the operating room table. At least one pelvic digital x-ray image of the patient positioned on the operating table is taken with the x-rays including the reference element. At least three points are identified on the intra-operative x-ray image. In the preferred embodiment, the points corresponding to the points on the pre-operative x-ray image. The lengths and angles between each of the at least three points on the intra-operative digital images are then calculated. An intra-operative angular deviation of the acetabular cup insertion instrument from the desired abduction and anteversion angle i.e. calculated by comparing the dimensional differences between the points on the pre-operative standing x-ray images and the at least one intra-operative x-ray image. The insertion instrument is then aligned to a calculated angular position, based on the intra-operative deviation, using the navigation camera and a navigation tracker mounted on the insertion instrument.
As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body.